Modems are commonly employed for allowing a host computer to input and output data to other devices through a telephone line connected to the public switched telephone network (PSTN). Traditionally, modems allowed for only data communication in the form of encoded data or facsimile data. To allow for ordinary, voice telephone calls, a separate telephone is needed for direct connection into the PSTN.
More recently modems have been developed which incorporate voice telephonic features. One such device is a data/fax/voice modem. The data/fax/voice modem automatically answers an incoming telephone call received along a telephone line connected to the PSTN. The data/fax/voice modem then determines whether the received telephone call includes a data message, a facsimile message, or an ordinary voice message. If the incoming message is a data message, a data modem module of the data/fax/voice modem operates to receive and decode the data message for storage in a computer memory or other storages of a host computer such as a hard disk drive. If the incoming message is a facsimile message, a fax module of the data/fax/voice modem operates to receive and decode the facsimile image for storage within the memory of the host computer. Alternatively, a facsimile print-out device may be connected to the data/fax/voice modem for automatically printing the facsimile image, thus eliminating the need to store the facsimile image within the host computer. If the incoming message is a voice message, an answering machine module of the data/fax/voice modem operates to answer the incoming voice communication. Initially, the answering machine module of the data/fax/voice modem outputs a prerecorded greeting, then records a voice message in response to the greeting. An analog-to-digital converter means is typically included within the data/fax/voice modem for converting the incoming voice message to digital signals for storing in the memory of the host computer. Alternatively, the data/fax/voice modem may be connected to a conventional analog recording device, such as a tape recorder for recording the voice message. As can be appreciated, a variety of implementations and configurations are available.
To allow an operator to listen to received voice messages and to record the answering machine greeting, a telephone headset is typically provided. The telephone headset, which includes an earphone and a microphone, is connected directly into the modem. However, no capability is provided for receiving or placing a voice telephone call using just the headset, modem and host computer. To be able to place telephone calls or to converse directly with a calling party, a separate telephone is required for connection to the PSTN line.
FIG. 1 illustrates a set-up wherein a telephone is provided in combination with a modem. More specifically, FIG. 1 illustrates a set-up having a modem 10, a host computer 12, a telephone headset 14, and a separate telephone 16. As can be seen, telephone 16 is directly connected to a telephone line 18 interconnecting the PSTN and modem 10. In some systems, telephone 16 is directly connected into modem 10. However, even in such systems, internal circuitry bypasses the modem operations to provide only a direct interconnection between telephone 16 and telephone line 18.
Although the setup of FIG. 1 provides for both placing and receiving voice telephone messages, the setup is not ideal. For example, undesirable redundancy results from a need to provide both a complete telephone 16 and a separate telephonic headset 14. In particular, two sets of speaker and microphone mechanisms are required, one on the headset and one on the telephone. Moreover, much of the functionality of the telephone, including call initiation, dialing, repertory dialing and call answering, are duplicated within the modem and the telephone. Another disadvantage is that a large bulky telephone is required to conduct ordinary voice communications. Carrying such a large telephone may not be practical, especially for portable applications. Moreover, the physical size of a standard module telephone jack is difficult to incorporate into small form-factor modems and other devices. To avoid these and other disadvantages, it would be desirable to provide a mechanism which would allow modem 10, host computer 12 and headset 14 to emulate the telephone, thereby eliminating the need to provide a separate telephone.
Similar disadvantages occur with other communications devices, besides modems, such as facsimile machines and personal digital assistant systems (PDA's).
Non-land-line based communication media have been employed for transmitting telephonic communications. Examples include cellular radio and related devices. Other examples include non-hard-wired local area networks (LAN's) which may employ radio or infrared signaling. The above-described disadvantages in conventional modems can occur in non-land-line based devices and are not limited solely to land based telephone lines or to the PSTN telephone network.
A natural extension of telephonic communication which has gained wide usage in recent years is the speakerphone. Using the speakerphone, a user can communicate without the cumbersome handset. Additionally, using the speakerphone, more than one user may join in a conversation at any given time. Consequently, the speakerphone offers great conveniences for users who require the use of a telephone while engaging in other tasks. The speakerphone is also ideal for teleconferences. Speakerphones are broadly classified as Half-Duplex or Full-Duplex. Half-Duplex speakerphones allow a conversation to proceed in one direction at a time. This one way at a time conversation keeps the speaker signal from being fed back into the microphone. On the other hand, Full-Duplex speakerphones allow a conversation to proceed in both directions simultaneously. This provides the most natural feel since it is just like using a telephone handset.
In general, the speakerphone requires more microphone sensitivity and more audio speaker drive than its counterpart, the more conventional handset telephone. The increase in microphone sensitivity and audio speaker drive, however, can exacerbate the problem of speaker-microphone coupling which may adversely effect the performance of the speakerphone. In short, output from the audio speaker can undesirably feed back into the microphone to form an infinite loop. If the feedback signal is slightly out of phase with the original microphone input signal and the loop gain is more than one (1), a system oscillation may occur thereby generating an echoing or howling sound that is analogous to holding the microphone of a public announce (PA) system directly in front of the system.
In Japanese Patent Application Laid-open 62-116023, a two-wire speakerphone system with an echo canceling circuit is disclosed. The echo canceling circuit utilizes current feedback to eliminate echoing or howling problems associated with the operation of a speakerphone. In telephone systems, two-wire circuits having a single pair of separate tip and ring lines, as opposed to four-wire circuits, are used to connect a telephone to a central switching office. This connection is called a local loop which carries a direct current supplied by batteries at the central switching office. The aforementioned direct current acts as a medium to allow speech and signaling communications to be transferred over the telephone network. The speakerphone system with echo canceling from Japanese Patent Application Laid-open 62-116023 is designed for a two-wire telephone circuit and uses negative feedback current in its echo canceling circuit to minimize echoing.
On the other hand, four-wire circuits in which two wires are used for transmitting and two for receiving are used in other telephone circuits such as the telephone handset and modem interface circuits. Four-wire circuits are used when all types of transmissions (i.e., digital data and voice) are transferred over the telephone network. Moreover, while four-wire circuits are designed for low voltages, two-wire circuits are designed for use with high voltages. In particular, two-wire circuits of telephone lines occasionally are known to carry up to 1500 volts when they come in contact with a live high-voltage power line. As a result, for safety and other reasons, Federal Communications Commission (FCC) Part 68 Specification for the telephone system requires that circuits and devices that are connected to high voltage portions of the telephone line such as two-wire circuits must be able to withstand a high-voltage exposure of up to 1500 volts. This requirement greatly effects the design, construction, and integration of circuits and devices connecting to high voltage portions of the telephone line. Because the echo canceling circuit from Japanese Patent Application Laid-open 62-116023 is designed for high voltages, its design and construction present technical and integration challenges particularly with its implementation in an integrated semiconductor chip. Moreover, the current feedback characteristic of the echo canceling circuit from Japanese Patent Application Laid-open 62-116023 presents a challenge to implement efficiently and inexpensively in a four-wire telephone circuit.